Portable electronic devices usually include keyboards or keypads so that information may be inputted to realize information exchange. When portable electronic devices are used in the dark, the keys need to be illuminated. Each individual key may be illuminated by a respective light source located directly behind the key. This arrangement is, however, very expensive since a separate light source is required for each key. Also, without carefully designed diffusers in or behind the keys, unsightly bright spots may be seen through the keys coinciding with the light source.
A more economical approach utilizes fewer light sources with the light being directed toward the keys with the aid of a light guide in the form of a transparent plate. A known light guide includes an array of apertures. A portion of each key depends or extends into the corresponding aperture so that light can be directed into the key from the light guide. Thus, some of the light is directed toward the apertures and so illuminates the keys. A drawback with this arrangement is that it does not give uniform illumination, because the lighting effect deteriorates toward the edges of the keyboard or keypad and other locations further away from the light sources.
An example of a light guide is disclosed in U.S. Pat. Nos. 5,083,240 and/or 5,053,928. The light guide is for a mobile phone, and defines a regular array of apertures which correspond to shaped keys. Four prismatic indentations are defined among the apertures. Each lens is disposed under each indentation in one side of the light guide, and is formed integrally with the light guide. Two square holes are provided in the middle of the light guide. Light is introduced into the light guide from distributed light sources such as light emitting diodes (LEDs) under each corresponding prismatic indentation. Each lens serves to collimate the majority of light from its associated light source into a light beam through the light guide. The indentations and the two square holes are arranged for reflecting light in a localized path substantially surrounding an area of each shaped key. Although the design of the light guide may illuminate the keys, the light guide requires more than one light source, which is an added cost and consumes more power.
Therefore, a new light guide is desired in order to overcome the above-described shortcomings.